A dry process for removing sulfur dioxide from a gaseous mixture containing sulfur dioxide is described in U.S. Pat. No. 4,626,418, the contents of which are incorporated by reference herein. In that process, the gaseous mixture is contacted with a mixture of at least one alkaline earth compound, such as calcium or magnesium carbonate, hydroxide or oxide, and from about 0.01 to 12 weight percent of a sugar.
While the process described in U.S. Pat. No. 4,626,418 gives an appreciable increase in the removal of sulfur dioxide from flue gases due to the presence of sugar in the alkaline earth additive, when the flue gases are subsequently directed to an electrostatic precipitator, a high resistivity of the particles leaving the boiler system impairs the ability of the electrostatic precipitator to produce a clear, or unopaque, discharge.
It has previously been proposed, in U.S. Pat. No. 4,519,995, to increase the relative moisture of a flue gas, such as by passing the same through a heat exchanger and cooling the gas and/or by spraying water into the flue gas prior to passing the gas into a filter system. That system uses a dry additive, either fine-grain calcium oxide or calcium carbonate in the coal combustion chamber, and the cooling or water addition is effected between a boiler and a dust filter, with clean gas dicharged through a stack following the filter. The provision of the dust filter after the firing is said to improve the binding rate of sulfur with the ashes, while the reactivity of sulfur is said to be further improved when the relative moisture of the flue gas is increased. The normal temperature of the gas is described as about 150.degree. C. (300.degree. F.), with the increase in the relative moisture reducing the temperature to a value between 50.degree. and 100.degree. C. (122.degree. and 212.degree. F.).
Another dry process for reducing the sulfur control of a flue gas is described in U.S. Pat. No. 4,670,238, wherein a dry sorbent is injected into the ductwork after a boiler. Calcium hydroxide, magnesium hydroxide, hydrated lime, or mixtures, and recycle sorbent are contacted with the flue gas following a preheater of a boiler. Downstream of the sorbent addition location, a humidifier is used to spray a solubilizing solution into the flue gas. The product stream is passed to a classifying electrostatic precipitator (ESP), from which unreacted lime as CaOH.sub.2 is ground prior to recycle for addition as recycle sorbent.
In U.S. Pat. No. 4,645,652, a combined removal of SO.sub.x and NO.sub.x is disclosed, at two temperature levels, with additives injected into flue gas ductwork leading from a boiler to a particulate collection device such as a fabric filter baghouse or an electrostatic precipitator. A dry alkaline powder is first dispersed into the ductwork to remove NO.sub.x and some SO.sub.x at a temperature of 250.degree. to 500.degree. F. and subsequently an aqueous alkali slurry or humidification water is injected into the ductwork to remove SO.sub.x from the flue gas. A similar system is disclosed in U.S. Pat. No. 4,645,653 wherein a spray absorber is provided. In that process, NO.sub.x is removed from flue gas by adding a dry alkaline powder to the ductwork upstream from a spray absorber and SO.sub.x is removed from the flue gas by contact with an alkaline sorbent.
It is an object of the present invention to provide an improved sulfur dioxide removal process for treating flue gases with a calcium hydroxide - sugar mixture which will produce a clear discharge from an electrostatic precipitator of the system, further improve the sulfur dioxide removal, and reduce the exhaust of oxides of nitrogen into the air from the system.